Human calumenin localizes to the secretory pathway and is secreted to the medium

Calumenin belongs to a family of multiple EF-hand proteins that include ret iculocalbin, ERC-55, and Cab45. Reticulocalbin and ERC-55 localize to the E R due to a C-terminal HDEL retrieval signal. Cab45 contains a HEEF C-termin al sequence and is localized to the Gels apparatus. The murine homologue of calumenin is reported to be present in the ER due to a C-terminal HDEF ret rieval signal. The human homologue differs from the murine at 7 amino acid positions but the HDEF signal is conserved. However, in the cultured human cell. lines, HaCaT keratinocytes, normal and transformed MRC-B fibroblasts, as well as in transfected COS-l cells, human calumenin could be demonstrat ed in the ER as well as in the Golgi complex. Especially in MRC-B cells, a certain heterogeneity was observed, with some of the cells having calumenin localized solely to the ER while in other cells calumenin could be demonst rated in the ER as well as in the Gels complex. Immunoelectron microscopy o f placental syncytiotrophoblast cells showed that a substantial fraction of calumenin is localized in close association with the ER membrane. In addit ion, the protein may be recovered from the medium of cultured cells in an e ndoglycosidase H-resistant form, suggesting that the glycosylated protein h as been further modified in the Gels apparatus and secreted to the medium. (C) 1999 Academic Press.